1. Field of the Invention
The present invention relates to a novel optically active compound, a photoreactive chiral agent for changing the helical structure of a liquid crystal, a liquid crystal composition, an optical film, a liquid crystal color filter, a recording medium, a method for changing the helical structure of a liquid crystal, and a method for fixing the helical structure.
2. Description of the Related Art
Recently, attention has been paid to a liquid crystal material such as a cholesteric liquid crystal which has a helical structure, and shows a variety of selectively reflected colors due to the helical twisting power (twist angle) of the helix. Furthermore, since the material is excellent in terms of the selective reflection property and the color purity of the selective reflection light, it is used widely for an optical filter, a liquid crystal color filter, a recording medium, or the like.
For example, a color filter used in a color liquid crystal display, or the like, is generally comprised of red (R), green (G), and blue (B) pixels, and a black matrix for improving the display contrast between the pixels. This type of color filter is conventionally one having a pigment dispersed in a resin, or one in which a dye is used. Methods for producing the color filter are usually applied in which a colored resin liquid is coated on a glass substrate by spin coating, or the like to thereby form a colored resist layer and patterning by a photolithography process is then carried out to form a color filter pixel, or a colored pixel is directly printed on a substrate.
However, in the manufacturing method of a color filter using a printing method, there are drawbacks in that the pixel resolution is low and it is difficult to produce a high resolution image pattern. In the manufacturing method using the spin coat method, there is a large amount of material loss, and in the case where a substrate having a large area is coated, there is the disadvantage that uneveness of the coat is large. Further, in a method of manufacturing using an electrodeposition method, a color filter can be obtained which has a comparatively high resolution and the uneveness of the color layer is relatively small, but there are problems with this method since the manufacturing process is complicated and managing the solutions is difficult.
From the above-mentioned reasons, there is demand for a method of producing a color filter in which a high quality color filter can be efficiently and easily produced with little material loss.
On the other hand, the color filter is required to have high transmittance and high color purity. Recently, improvement of color purity and transmittance have been achieved by optimizing the kind of the dye or the coloring resin. Also, in a method using a pigment, transmittance and purity may be improved by using a fine pigment which is thoroughly dispersed. However, recently the demand for the transmittance and the color purity of the color filter in a liquid crystal display (LCD) panel has been extremely high. In particular, in the case of a color filter for a reflection type LCD, it is difficult to simultaneously realize a degree of whiteness like paper white, high contrast and color reproduction. In contrast, in the case of a color filter produced by coloring a resin with a dye, or by dispersing a pigment in a conventional production method, since both are a light absorbing type color filters, improvement of the color purity by further improving the transmittance has substantially reached to the limit.
A color filter which was polarized light and has a cholesteric liquid crystal as the main component is known. Since this type of color filter displays an image by reflecting a predetermined amount of light and transmitting the rest, the efficiency of light use is high and performance in terms of the transmittance and color purity is superior to that of the light absorbing type color filter. On the other hand, from the viewpoint of obtaining even thickness, a method of forming a film on a substrate using a spin coat process, or the like has been commonly used as the production method therefor. However, in this method, there is a large amount of material loss, and thus it is disadvantageous in terms of the cost.
As a means for solving the above-mentioned problems and for ensuring uniformity in the color filter film (e.g. uniform color purity), and further in order to realize reduction of the number of production steps, a method of using a photoreactive chiral compound is useful. This method is based on the principle that when a liquid crystal composition including a photoreactive chiral compound is irradiated so as to form a pattern with light of the reactive wavelength of the chiral compound, since the reaction of the chiral compound proceeds in accordance with the intensity of the radiation energy so as to change the helical pitch (twist angle of the helix) of the liquid crystal compound, a selectively reflected color is formed for each pixel just by pattern exposure of differing light amounts. That is, this method is advantageous in that the many patternings for the color filter formation can be completed by carrying out mask exposure only once using masks with different transmissible light amounts.
Therefore, by fixing a cholesteric liquid crystal compound after image-wise irradiation of light and patterning, a film capable of functioning as a color filter can be formed. This can also be applied for an optical film, an image recording, or the like.
In particular, in the case of producing a color filter by carrying out mask exposure once, it is desired that the three primary colors which are B (blue), G (green), and R (red) are formed with a good color purity by carrying out exposure only once. However, when the liquid crystal twist change rate is small, a sufficient color purity cannot be obtained. Therefore, from the viewpoint of displaying the three primary colors with a high color purity by carrying out exposure once, it is necessary to use as a photoreactive chiral compound a chiral compound (chiral agent) having a large twist change rate, and capable of significantly changing the helical twisting power of the helical structure of the liquid crystal compound. That is, by using a chiral compound having a large twist change rate, the range of the hues which display selective reflection can be expanded by changing the radiation amount.
The present inventor has applied a photoreactive chiral agent (optically active compound) whose photoreactivity is such that it is capable of changing the liquid crystal helical twisting power (twist angle) in accordance with the amount of light irradiated. Further the helical twisting power (twist angle) change ratio is large and thus a wide range of a wavelength region is capable of various selective reflection particularly in the case of a cholesteric liquid crystal phase. This photoreactive chiral agent is also capable of displaying the three primary colors (B, G, R) with a high color purity (Japanese Patent Application Nos. 2000-193142, 2000-193143). However, development of other different types of chiral compounds has been anticipated.
According to  less than 21 greater than , page 327 of xe2x80x9cLiquid Crystalsxe2x80x9d 1996, and  less than 24 greater than , page 219 of the same 1998, the HTP (helical twisting power) can be enlarged by introducing a substituent at the 6th position and the 6xe2x80x2th position of a 2,2xe2x80x2-methylenedioxy-1,1xe2x80x2-binaphthol. However, the substituents at the 6th position and the 6xe2x80x2th position have a structure different from those of the substituents at the 6th position and the 6xe2x80x2th position of an optically active compound of the present invention which is described later. Moreover, no description is given therein of photoreactivity.
The present invention has been achieved in view of the above-mentioned problems with the conventional products. An object thereof is to provide: an optically active compound useful as a photoreactive chiral agent which is capable of the photoisomeriation and significantly changing the helical structure (or helical twisting power, twist angle) of the liquid crystal by the light irradiation when it is added to a liquid crystal compound; a photoreactive chiral agent including the above-mentioned optically active compound; a liquid crystal composition with the above-mentioned optically active compound added; a liquid crystal color filter; an optical film; and a recording medium. Furthermore, another object of the present invention is to provide a method for changing the helical structure of the liquid crystal of a liquid crystal composition including the above-mentioned optically active compound, and a method for fixing the helical structure.
A first aspect of the present invention is an optically active compound represented by the general formula (1). 
(wherein, Ra and Rb each independently denotes a hydrogen atom, an alkyl group, an aryl group, a heterocyclic group, an alkenyl group, or an alkynyl group, Rc and Rd each independently denotes a hydrogen atom, an alkyl group, or an alkoxycarbonyl group, and L denotes a divalent group. A binaphtyl portion has an axial asymmetry of either (R) or (S).)
Moreover, a second aspect of the present invention is a photoreactive chiral agent including the above-mentioned optically active compound.
Furthermore, a third aspect of the present invention is a liquid crystal composition containing at least a liquid crystal compound and the above-mentioned optically active compound.
Moreover, a fourth aspect of the present invention is a liquid crystal composition containing a liquid crystal compound having at least one polymerizable group, a photo polymerization initiator, and the above-mentioned optically active compound.
Furthermore, a fifth aspect of the present invention is a method for changing the helical structure of the liquid crystal by changing the structure of the above-mentioned optically active compound by irradiating a light to the liquid crystal composition according to the above-mentioned third or fourth aspect.
Moreover, a sixth aspect of the present invention is a method for fixing the helical structure of the liquid crystal comprising the steps of image-wise irradiating light of the photosensitive wavelength region of the above-mentioned optically active compound to the liquid crystal composition according to the fourth aspect, and irradiating light of the photosensitive wavelength region of the above-mentioned photo polymerization initiator so as to carry out the photo polymerization.
A seventh aspect of the present invention is a liquid crystal color filter containing at least a liquid crystal compound and at least one of the above-mentioned optically active compounds.
An eighth aspect of the present invention is an optical film containing at least a liquid crystal compound and at least one of the above-mentioned optically active compounds.
A ninth aspect of the present invention is a recording medium containing at least a liquid crystal compound and at least one of the above-mentioned optically active compounds.